1. Technical Field
This invention relates generally to the use of signal processing techniques for determining the transmission performance data of cabling systems, and more particularly, to techniques for canceling undesired signals in cabling systems.
2. Background of the Invention
The transmission performance characteristics of modern high speed data communication twisted pair cabling systems are defined by various international and industry working bodies (standards organizations) to assure standard data communication protocols can successfully be transmitted through the transmission media. These data communication cabling systems (known as channels) typically consist of connectors (modular 8 plugs and jacks) and one or more forms of twisted pair cabling. The requirements for important RF transmission performance parameters such as Near End Crosstalk (NEXT), Return Loss, Insertion Loss, and Equal Level Far End Crosstalk (ELFEXT) are typically specified as a function of frequency. To assure compliance of cabling systems with these requirements, field test instruments are available to certify that installed cabling meets the frequency domain requirements. These instruments perform certain measurements to verify compliance with the standards and provide an overall Pass or Fail indication.
A typical channel 100 in a structured cabling system and the associated field test configuration is shown in FIG. 1. The channel 100 consists of a user""s patch cord 102 (typically connecting a network hub to a patch panel), a data communication patch panel 104 (generally located in a wiring closet), a length of cable 106, data connectors 108a,b, and another patch cord 110 between connector 108 and a user""s computer/workstation (not shown).
Field testing of the channel transmission performance is typically done with field test equipment that runs a full suite of frequency domain tests from both ends of the channel. The field test equipment is typically interfaced through a xe2x80x9cchannel adapterxe2x80x9d containing a modular-8 jack to connect to the user""s patch cords on both ends of the channel. Tests of Near-End Crosstalk (NEXT), Return Loss, Insertion Loss (Attenuation), and ELFEXT (Equal-Level Far-End Crosstalk) are the typical measurements performed by these instruments to certify transmission parameters. The measurements are then compared to a set of limit criteria as defined by the specific category of performance and a Pass/Fail indication is made.
An example of a NEXT measurement for a high performance Category 6 channel is shown in FIG. 2. If the NEXT value exceeds the limit, then it is considered to have failed, and its data transmission capability is questionable. Field testers perform similar tests for the other transmission parameters of a channel to qualify the channel for use.
One field test problem arises from the definition of the components of the cabling that are to be included in the channel field test measurements. Referring back to FIG. 1, channel 100 is defined as xe2x80x9cnearxe2x80x9d user patch cord 102, a pair of connections 114, 116, the length of xe2x80x9chorizontalxe2x80x9d cable 106, and xe2x80x9cfarxe2x80x9d end user patch cord 110. However, this channel definition does not include first and last connections 108a,b where field tester 118 (and ultimately computer network equipment) are interfaced. The excluded components include the modular jack on the test equipment and the modular plug that is on the end of the user patch cord. This definition may be problematic as measurements must necessarily be made through these connections. It is impractical (and unpopular) to cut off a connector from the user""s patch cord to make the test as it is often done in laboratory environments. The crosstalk, return loss and other effects of these connections must instead be ignored (cancelled or otherwise suppressed) when measuring the channel. These connections can have a considerable affect on the measurement and mask the true performance of the channel under test. Often the contributions of these connections will cause an otherwise compliant link to fail. Accordingly, it is important that channel performance be measured accurately per the definition of the appropriate standard to ensure its transmission capability. Currently this channel measurement problem is not addressed adequately by any field test equipment.
In current implementations of field test instruments, several methods are employed to make measurements through these necessary but troublesome connections. The first and simplest is to simply accept the additional crosstalk, return loss, etc. of the instrument connections and include them in the accuracy specification of the test instrument. This may result in an inaccurate measurement of channel 100. Instruments that do not correct for the instrument connections are generally classified by the standards body as having xe2x80x9cLevel I Accuracyxe2x80x9d due to errors introduced by measuring through instrument connections with no compensation. Level I Accuracy instruments are generally not considered adequate for testing of new, high performance cabling systems.
A second, seemingly improved method for suppressing the instrument connections is to subtract out the contribution of a xe2x80x9cnominalxe2x80x9d connection from the measured data. This method requires that the measurement instrument have full knowledge of the magnitude and phase of both the measured signals and connection characteristics. While this method may appear to be a solution, in fact, there can be significant differences between the amount of crosstalk, return loss, and the like contributed by the specific jack and plug comprising the connection. The variability often results in tradeoffs made by each manufacturer and physical issues associated with terminating a plug onto a cable. Plugs from the same manufacturer that appear identical can have significantly different NEXT characteristics. Therefore, it may not be possible to have a priori knowledge of a suitable connection contribution that must be cancelled. That is, subtraction of xe2x80x9cnominalxe2x80x9d crosstalk from measured data can increase the amount of indicated crosstalk, return loss and the like. Thus, it is unlikely that test equipment utilizing this method of correction, known as fixed vector cancellation, could qualify for better than Level I performance.
Another method that at first consideration seems to be effective for improving response measurements is called xe2x80x9ctime gatingxe2x80x9d. An example of this technique is disclosed in U.S. Pat. No. 5,698,985, entitled xe2x80x9cCross-Talk Measurement Instrument With Source Indication as a Function of Distance,xe2x80x9d issued to Bottman and assigned to the Fluke Corporation on Dec. 16, 1997. The channel measurements are performed through and including the channel adapter and user""s cord connector and these results are converted to the time domain. A section of the time data corresponding to the location of the instrument connection is then mathematically set to zero (i.e. xe2x80x9ctime-gated outxe2x80x9d), and the modified time data is converted back into the frequency domain for comparison with channel performance limits. This method has the benefit that the contribution of crosstalk, return loss and the like, of the instrument connection may be completely suppressed without having prior knowledge of the connection characteristics of performance.
However, this method suffers from additional shortcomings. For example, one problem with time gating results from the fact that there is limited bandwidth available in the frequency domain, and thus limited time resolution to perform the time gating. In order to minimize other artifacts related to the time gating procedure, the amount of time data that must be zeroed or otherwise modified must necessarily extend well beyond the instrument connection.
A second limitation of the time gating method primarily affects the return loss measurement. Return loss is a measure of the amount of signal reflected by the transmission channel. Reflections at the beginning of the channel have two significant components. One reflection is from xe2x80x9cpoint sourcesxe2x80x9d of returned signal, such as the connector interface. The second reflection is caused by the transition from the impedance of the test environment to the impedance of the cable. Time gating for return loss measurements can lead to significant errors due to ignoring the initial cable impedance mismatch as well ignoring reflections in the first few feet of the cable. These initial sources of return loss can sometimes be a dominant contributor to a xe2x80x9cFAILxe2x80x9d return loss indication and preferably should not be suppressed. Time gating results in missing an important portion of the crosstalk or return loss in the user""s patch cord. Current field testers that implement some form of time gating typically ignore up to the first 3 to 4 feet of the user patch cable due to bandwidth and sampling considerations. This can be a serious shortcoming since the channel high frequency performance is strongly influenced by the first few feet of the cable. The xe2x80x9cdead zonexe2x80x9d created by the time gating does not adhere to the true definition of the channel which starts immediately after the plug. Bandwidths in the 10s of GHz are typically required in order to have a suitably short dead zone so as not to impact the measurement. This extremely wide bandwidth requirement is beyond the current state of field test equipment.
A method of signal correction in accordance with the present invention, may be referred to as an Adaptive Vector Cancellation method (AVC). In an exemplary embodiment of the present invention, with AVC, time domain data for an instrument connection is obtained and an estimate is made of the magnitude, phase, and time position of a signal response such as NEXT or Return Loss associated with the connection. For example, for a channel adapter whose connector is close to the instrument itself, the connection response occurs near time zero. Based on the estimate of the amplitude and time of the connection response, a suitable full-bandwidth frequency response that corresponds to a point source of NEXT or Return Loss is calculated, referred to in a table or determined through other similar means. This calculated connector response is then scaled to an appropriate magnitude, phase and time shifted to the estimated position of the actual connection. The scaled/shifted response is then vectorially combined with the measured sweep data to suitably cancel the connection contribution to NEXT and/or Return Loss. Thus, the amount of NEXT or return loss existing in the user""s patch cord is preserved, while the NEXT or return loss due to the instrument connection (which is essentially a xe2x80x9cpoint sourcexe2x80x9d) is suitably suppressed. Correction is done in the frequency domain, over the full bandwidth of the measured data.
AVC combines time domain analysis and non-band-width-limited application of that analysis in a manner that results in a channel measurement that more closely meets or exceeds industry requirements for measurement accuracy. Current field test instruments use time domain techniques to identify and time-gate out sources of NEXT and return loss in, but they do not use this information to estimate the magnitude, phase and position of these sources so they can be cancelled. Using AVC for channel measurement processes the time data to estimate the magnitude, phase and exact location of the channel adapter NEXT or return loss and then subtracts the idealized frequency domain model of those sources at the correct position, magnitude and phase.
Further, AVC may be suitably applied to measurements where the connection to the cabling system is removed by an extension of a length of cable or other media. In such a system, the crosstalk and return loss influence of that extension by conventional means, then the crosstalk and return loss contribution of the connector interface (e.g. to the channel) at the end of the extension may be suitably removed.
The development of a field test instrument that can make accurate channel measurements that comply with standards is beneficial to the data communication industry. Up to the time of this invention there has been no method for making field channel measurements that are accurate and compliant with the requirements of the industry standards.